Polyurethane prepolymers containing terminal isocyanate groups have been known for some time. They can readily be reacted with suitable hardeners--generally polyhydric alcohols--to form high polymers. Polyurethane prepolymers have acquired significance in numerous fields, including for example sealing compounds, lacquers and adhesives.
To obtain polyurethane prepolymers containing terminal isocyanate groups, it is standard practice to react polyhydric alcohols with an excess of diisocyanates. It is generally known in polymer chemistry that, in this reaction, the molecular weight can be at least approximately controlled through the ratio of hydroxyl groups to isocyanate groups. Thus, products of very high molecular weight are formed where the molar ratio is exactly 1:1, whereas, on a statistical average, adducts of 2 molecules of isocyanate with 1 molecule of diol are formed where the molar ratio of OH to NCO is 1:2. On the strength of this knowledge, it is possible for those skilled in this art to custom-make polyurethane prepolymers containing terminal isocyanate groups with average molecular weights varying within wide limits. However, the products formed show a more or less wide molecular weight distribution, as is normally the case with polymers. In particular, a certain amount of the component which is used in excess remains unreacted at the end of the reaction, irrespective of the reaction time. At the same time, the content of unreacted diisocyanates, referred to herein as residual monomers, increases with the excess of this component in the reaction mixture (cf. H. G. Elias, Makromolekule, Huthig & Wepf, Basel, 4th Edition 1981, pages 487 et seq.).
In numerous applications of polyurethane prepolymers, problems are presented by the presence of residual monomers. Thus, volatile diisocyanates, such as tolylene diisocyanate for example, necessitate particular precautionary measures on an industrial hygiene level, even when they are present in the prepolymers in quantities of only 0.5 to 5% by weight. On the other hand, involatile diisocyanates can cause problems through migration in the field of bonding. Problems such as these can arise, for example, in the sealing of bonded film-to-film laminates.
To reduce the residual monomer content, it is possible in the case of volatile diisocyanates to remove these diisocyanates from the prepolymers by thin-layer distillation under reduced pressure and at an elevated temperature in the range of from about 80.degree. to 150.degree. C. However, this process is complicated and occasionally gives unsatisfactory results. In addition, it is confined to volatile diisocyanates. In the case of nonvolatile diisocyanates, such as the dicyclic diisocyanates, distillation has little to offer. Accordingly, products of relatively high molecular weight are generally used; products such as these having a relatively low residual monomer content because of the theoretical relationships explained at the beginning. Since they also have relatively high viscosities, it is standard practice to use the prepolymers in solution in organic aprotic solvents. However, the use of organic solvents is ecologically unfavorable and is no longer acceptable in numerous applications.